Submarines typically comprise a pressure hull and a sail extending vertically upwards from the pressure hull, both of the pressure hull and sail typically being submerged in water below a water to air interface in normal use. The sail typically comprises a chamber storing one or more deployable masts, such as telecommunications antennae, an air induction tube for drawing in air for an internal combustion process in a diesel engine and an exhaust tube for the diesel engine. In order to use (at least) these masts, the submarine rises close to the water surface in a “snorkelling mode” before deploying the mast such that at least part of the mast protrudes above the water surface.
The mast comprising the air induction tube has an upper portion which protrudes above the water surface when the submarine is in snorkelling mode. The said upper portion comprises an air inlet port for receiving air into the air induction tube from above the water surface. The air inlet port on the mast is typically always open, and a snorkel valve is provided inside the mast, adjacent to the air inlet port, to control the ingress of water into the air induction tube. Typically the snorkel valve is a butterfly valve comprising a disk mounted on a rotatable stem, the disk being pivotable about a pivot axis defined by the longitudinal axis of the stem between a closed position and an open position. In the closed position, the disk covers the valve inlet and engages an annular gasket extending around the perimeter of the valve inlet. In the open position, the disk is pivoted about the pivot axis so as to disengage the gasket, and uncover a portion of the valve inlet.
Although butterfly valves are an established technology, they have a number of disadvantages. The shaft needs to be sealed (particularly when the disk is in the valve closed position) and, as the shaft is moveable (rotatable about the pivot axis), this seal is prone to wear. Furthermore, even when the disk is in the fully open position, the disk and the shaft cover a portion of the inlet, thereby restricting flow through the inlet. In addition, obstructions between the disk and the gasket can prevent the disk from returning to the closed position from the open position.